Isocyanates are produced in large volumes and serve mainly as starting materials for production of polyurethanes. They are usually prepared by reacting the corresponding amines with phosgene, using phosgene in a stoichiometric excess. The reaction of the amines with the phosgene can be effected either in the gas phase or in the liquid phase. It is a feature of the process regime in the gas phase, typically referred to as gas phase phosgenation, that the reaction conditions are chosen such that at least the amine, isocyanate and phosgene reaction components, but preferably all the reactants, products and reaction intermediates, are gaseous under the conditions chosen. Advantages of gas phase phosgenation include reduced occurrence of phosgene (called phosgene “hold-up”), the avoidance of intermediates that are difficult to phosgenate, increased reaction yields and a lower energy requirement, since less solvent is being employed. The present invention relates exclusively to gas phase phosgenation and relates especially to a seamless method of starting up a gas phase phosgenation plant.
The prior art discloses various processes for preparing isocyanates by reacting amines with phosgene in the gas phase. An important factor for a good process regime is good mixing of the reactants of the gas phase phosgenation. EP-A-0 289 840 describes the preparation of diisocyanates by gas phase phosgenation, wherein the preparation in accordance with the invention takes place in a turbulent flow at temperatures between 200° C. and 600° C. in a cylindrical space without moving parts.
EP-A-0 570 799 relates to a process for preparing aromatic diisocyanates, in which the reaction of the corresponding diamine with the phosgene is conducted in a tubular reactor above the boiling temperature of the diamine within a mean contact time of 0.5 to 5 seconds.
EP-A-0 699 657 describes a process for preparing aromatic diisocyanates in the gas phase, in which the reaction of the corresponding diamine with the phosgene takes place in a reactor comprising two zones, wherein the first zone, which makes up about 20% to 80% of the total reactor volume, has ideal mixing and the second zone, which makes up 80% to 20% of the total reactor volume, can be characterized by plug flow. Preferably, the second reaction zone is executed as a tubular reactor.
The optimization of the use of tubular reactors for gas phase phosgenation, the principle of which has been disclosed in EP-A-0 570 799 with use of the jet mixer principle (Chemie-Ing.-Techn. 44 (1972) p. 1055, fig. 10), is the subject of numerous applications.
According to the teaching of EP-A-1 362 847, homogenization of the reactant stream supplied via the ring space of the tubular reactor and very central feeding of the two reactant streams into the tubular reactor have a great positive influence on the stability of the reaction zone and hence on the gas phase reaction overall.
As described in EP-A-1 555 258, enlargement of the tubular reactors used also necessitates enlargement of the mixing nozzle, which frequently takes the form of a smooth jet nozzle. However, the increase in the diameter of the smooth jet nozzle also reduces the speed of mixing of the central jet as a result of the greater diffusion length required and increases the risk of backmixing, which in turn leads to the formation of polymeric impurities and hence of solid material baked onto the reactor. According to the teaching of EP-A-1 555 258, the disadvantages described can be eliminated when one reactant stream is injected at high velocity via a concentric annular gap in the stream of the other reactant. This makes the diffusion length for mixing small and the mixing times very short. The reaction can then proceed with high selectivity to give the desired isocyanate. The occurrence of polymeric impurities and the formation of caked-on material are reduced thereby.
According to the teaching of EP-A-1 526 129, an increase in the turbulence of the reactant stream in the central nozzle has a positive influence on the mixing of the reactants and hence on the gas phase reaction overall. As a result of the better mixing, there is a decrease in the tendency to form by-products.
EP-A-1 449 826 discloses a process for preparing diisocyanates by phosgenation of the corresponding diamines, in which the vaporous diamines, optionally diluted with an inert gas or with the vapors of an inert solvent, and phosgene, are heated separately to temperatures of 200° C. to 600° C. and mixed and reacted in a tubular reactor, wherein a number n≧2 of nozzles aligned parallel to the axis of the tubular reactor are arranged within the tubular reactor, wherein the stream comprising the diamines is fed to the tubular reactor via the n nozzles and the phosgene stream is fed to the tubular reactor via the remaining free space.
A further development of the use of tubular reactors for gas phase phosgenation is the subject of WO 2007/028715. The reactant used has a mixing device and a reaction space. According to the teaching of WO 2007/028715, the reaction space comprises, in the front region, the mixing space in which predominantly the mixing of the gaseous phosgene and amine reactants, optionally mixed with inert medium, takes place, which is generally accompanied by the onset of the reaction. According to the teaching of WO 2007/028715, in the rear part of the reaction space, it is essentially only the reaction that then takes place, and mixing at most to a minor degree. Preferably, in the process disclosed in WO 2007/028715, reaction spaces that are rotationally symmetric with respect to the flow direction are used, it being possible to divide these, in terms of construction, essentially into up to four longitudinal sections along the longitudinal axis of the reactor over the flow profile, the longitudinal sections differing in terms of the size of the cross-sectional flow area.
WO 2008/055898 discloses a process for preparing isocyanates by phosgenation of the corresponding amines in the gas phase in a reactor, in which, analogously to WO 2007/028715, the reactor used has a mixing device and a reaction space, the rotationally symmetric reaction space can be divided, in terms of construction, essentially into up to four longitudinal sections along the longitudinal axis of the reactor over the flow profile, the longitudinal sections differing in terms of the size of the cross-sectional flow area. Compared to WO 2007/028715, the changes in the cross-sectional flow areas, however, are achieved not by means of a voluminous body installed into a tubular reactor but by means of a corresponding extension or constriction of the outer reactor wall.
EP-A-1 275 639 likewise discloses, as a possible process variant for preparation of isocyanates by phosgenation of the corresponding amines with phosgene in the gas phase, the use of a reactor in which the reaction space has, in flow direction, beyond the mixing of the two reactants, an extension of the cross-sectional flow area. By means of a suitably chosen extension of the cross-sectional area, it is possible to keep the flow rate of the reaction mixture over the length of the reactor just constant. This increases the reaction time available with the same reactor length.
EP-A-2 196 455 discloses that phosgene and the primary aromatic amines are converted above the boiling temperature of the amines in a reactor comprising a reaction space which is essentially rotationally symmetric with respect to the flow direction, wherein the cross-sectional average flow rate of the reaction mixture along the axis of the essentially rotationally symmetric reaction space in the section of the reaction space in which the conversion of the amino groups to the isocyanate groups is between 4% and 80% is not more than 8 m/sec and wherein the cross-sectional average flow rate of the reaction mixture along the axis of the essentially rotationally symmetric reaction space in this section of the reaction space is always below the cross-sectional average flow rate at the start of this section.
EP-A-1 935 876 discloses a gas phase process for preparing isocyanates by reacting corresponding primary amines with phosgene, in which phosgene and the primary amines are converted above the boiling temperature of the amines within a mean contact time of 0.05 to 15 seconds, the conversion being conducted adiabatically.
EP-A-2 408 738 discloses how a dissociation of phosgene to chlorine and carbon monoxide as a result of an excessively long residence time of the phosgene-containing streams at high temperature can be avoided. By reduction of the residence time of the phosgene at temperatures greater than 300° C. to a maximum of 5 s and by the limitation of the temperature of the heat transfer areas in contact with phosgene of not more than 20 K above the phosgene temperature to be established, this is said to be avoided.
EP-B-1 935 875 discloses a process for preparing isocyanates by reacting corresponding primary amines with phosgene in the gas phase, in which the reaction is stopped by conducting the reaction mixture out of the reaction space through a cooling zone into which liquids are injected, the direct cooling in the cooling zone being effected in one stage in two or more cooling zones connected in series (called “quenching” of the reaction mixture).
WO 2013/029918 describes a process for preparing isocyanates by reacting the corresponding amines with phosgene, which can also be conducted at different loads on the gas phase plant without any problems, and more particularly, even when running the plant in the partial load range, the mixing and/or the reaction is said to proceed within the optimized residence time window in each case, by increasing the ratio of phosgene to amine or adding one or more inert substances to the phosgene and/or amine stream. The method of the invention is to enable operation of an existing plant at different loads with constant product and process quality. This is to dispense with the provision of several plants with different nameplate capacities.
The application teaches that essential parameters of a phosgenation, such as the residence times of the co-reactants in the individual apparatuses in particular, are optimized for the operation of the production plant at nameplate capacity, which can lead to problems in terms of yield and product purity when the plant is operated at lower than nameplate capacity (cf. page 2 lines 20 to 36). In order to be able to attain the optimized—narrow—residence time window even at partial load (i.e. reduced amine flow rate compared to operation at nameplate capacity), it is suggested that either the phosgene stream and/or the inert fraction be increased (cf. page 3 lines 5 to 19), preferably in such a way that the total flow rate of all components corresponds essentially to that at nameplate capacity (cf. page 6 lines 4 to 8). The application does mention shut down operations in the description of the background of the invention claimed on page 2, but does not disclose any technical teaching at all as to the specific way in which a production plant in operation is most advantageously shut down (i.e. amine flow rate and phosgene flow rate are equal to zero). The technical measures disclosed in the application (i.e. the increase in the phosgene flow rate and/or the inert fraction) should be considered exclusively in the context of the problem of operation of a production plant at lower than nameplate capacity, and with the problem of how a plant operated at nameplate capacity can advantageously be switched to operation (i.e. amine and phosgene flow rate significantly greater than zero) at lower than nameplate capacity (see the examples).
Although the prior art processes described succeed in conducting a phosgenation without loss of quality in the end products, the only processes described, with a few exceptions, are those in the normal state of operation. There is no description of the shutdown operation, i.e. the safe shutdown of a gas phase phosgenation plant in a manner which enables seamless restarting at a later time.
The person skilled in the art is aware that such a continuously operated industrial process should not unnecessarily be abruptly shut down. The shutdown of a gas phase phosgenation plant is a frequent everyday industrial operation which need not necessarily be combined with opening or another mechanical intervention into the phosgenation plant. In practice, it is a feature of shutdown that there may be deviations in the excess of phosgene relative to amine compared to the continuous operation at the target nameplate capacity of the production plant. Such deviations occur particularly when, for example, pressure variations result in backmixing. This is observed especially when the current flow rate of amine to be converted is very small compared to the target flow rate of amine to be converted at the target nameplate capacity of the plant. These quantitative variations in the ratio of phosgene to amine are disadvantageous since solids such as polyurea or amine hydrochlorides can precipitate out. Furthermore, in the event of improper shutdown, there can be unwanted formation of droplets of amine. The shutdown of a gas phase phosgenation plant is therefore a critical process step, since errors here can seriously disrupt the later restart and the actual continuous production (for example as a result of an increase in the pressure differential needed to assure a sufficient flow rate of the reactants and products through the plant).